Syntheses Of SAPO-18 Zeolites With Excellent MTO Performance

Methanol to olefin reaction(MTO),as a nonpetroleum technology route for the efficient production of low-carbon olefins,has developed vigorously in recent years.Silicoaluminophosphate zeolites have been widely used in MTO reactions and have achieved industrial applications.However,these catalysts still have shortcomings such as adjusting difficultly the selectivity of light olefins and short lifespan.It is still a very important topic to explore highly stable functional zeolites with excellent MTO performance.SAPO-18 zeolite with AEI topology has been widely studied and reported due to its high propylene selectivity and relatively low coke deposition rate.In this paper,a series of SAPO-18 zeolite samples suitable for MTO reaction were rapidly synthesized by adjusting the crystal size,pore structure,and active sites of SAPO-18 zeolite through technical schemes such as adding heterocrystalline seeds,constructing multi-level pore structures,and kaolin assisted synthesis.The effects of gel ratio and heterocrystalline seed assisted synthesis on the grain size and active sites were studied,the formation mechanism of multistage pores and their regulation of reaction performance were investigated,the crystallization mechanism of rapid synthesis of SAPO-18 zeolite based on kaolin were explored.Focusing on exploring the relationship between the microstructure of mesoporous SAPO-18 zeolite and excellent MTO reaction performance,a series of regular understandings were obtained,which has an important guiding role in developing new MTO catalysts with adjustable olefin selectivity and long lifetime.The main research results were as follows:(1)The SAO-18 zeolite with submicron size was obtained by adjusting the initial gel ratio,and the SAPO-18 active site was further regulated by the induction of heterocrystalline seeds.It was found that the proportion of gel affected the crystal morphology of zeolite,and uniform cuboid crystals with size<500 nm were obtained after crystallization at 180℃ for 72 h.SAPO-34,as a heterocrystalline seed,promoted the crystallization of SAPO-18,however,when the crystallization time was less than 60 h,the formation of active sites in the sample could be inhibited.The submicron(<500 nm)SAPO-18 zeolite can accelerate the reaction mass transfer process,and its MTO catalytic life is three times than that of the sample prepared by the traditional gel ratio.(2)A multi-stage porous SAPO-18 zeolite with abundant intragranular mesopores was prepared using a cost-effective organic cationic nonsurfactant(polydimethyldiallylammonium chloride,PDADMAC)as a pore forming agent,a morphology regulator,and a silicon inducer using a one-step design method.It was found that PDADMAC could make the crystal tend to be lamellar growth,promoting the entry of silicon atoms into the zeolite framework,and optimizing the acidic environment of multi-level porous zeolite sheets.Compared with conventional submicron zeolites,the catalytic lifetime of multi pore SAPO-18 zeolites in MTO reaction has been increased by three times,and the selectivity for ethylene and propylene has been improved by 4%.(3)Using metakaolin as raw material,SAPO-18 zeolite with high skeleton silicon content and good dispersion was rapidly synthesized.The analysis and characterization results showed that during the crystallization process,the active aluminum and silicon in metakaolin participated directly in the synthesis of SAPO-18 in the form of primary structural units(PBUs).Furthermore,silicon in the form of Si(4Al)coordination is directly introduced into the framework of SAPO-18 zeolite to construct mild active sites,accelerating the crystallization rate of SAPO-18 zeolite.The SAPO-18 prepared after 2 h crystallization has the longest lifetime(320 min)in the MTO reaction,and the most yields of ethylene and propylene(72.9%).This research work has provided a new idea for low-cost and rapid synthesis of SAPO zeolites and construction of mild active sites.